1. Field of the Invention
The present invention relates to an image forming apparatus for forming a visible image by shifting toner to a latent image formed by a difference in electrostatic potential according to an electrophotographic system, electrostatic recording system, ionography or the like, and a fixing device for fixing a toner image, which has been transferred onto a recording medium, on the recording medium by heating and pressurization.
2. Description of the Related Art
A toner image formed by shifting toner to an electrostatic latent image on an image carrier is directly transferred onto a recording medium, or is once primary transferred onto an intermediate transfer member and is further secondary transferred from this intermediate transfer member onto a recording medium. For the transfer of the toner image, there is widely used a method in which a member for carrying an image is brought into contact with or is made to approach a member receiving a transferred image at a transfer portion, and an electric field is formed at this transfer portion to electrostatically shift the toner having an electric charge.
However, in the above method of electrostatically transferring the toner image, there is a case where scattering of the toner occurs, and resolution or dot reproducibility is lowered. Besides, in the case where toner images of plural colors are overlapped with each other and they are transferred at the same time, since transfer efficiency is not sufficient, there is also a case where uneven density or uneven color occurs in an image.
Under the circumstances, there is proposed a technique in which when toner on an intermediate transfer member is transferred onto a recording medium, a toner image on the intermediate transfer member is heated and melted, this is brought into press contact with the recording medium, and transfer and fixation are carried out at the same time.
Like this, the technique for carrying out the transfer and fixation at the same time can be classified according to methods and timing for carrying out heating and pressing as follows:
A first type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 2-106774, before a toner image on an intermediate transfer member is transferred, a recording member is heated, and toner on the intermediate transfer member is melted by heat of the recording member, and is transferred and fixed onto the recording member.
A second type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-15933, an endless belt shaped intermediate transfer member is overlapped with a recording medium, these are sent to a nip portion where a heating body and a pressure member are pressed against each other, and heating and pressurization are carried out.
A third type is such that, for example, like a technique disclosed in Japanese Patent Unexamined Publication No. Hei. 11-352804, a toner image is heated on an endless belt shaped intermediate transfer member, and the melted toner image is brought into press contact with a recording medium in a non-heating state.
Among the techniques as described above, in an apparatus of the third type in which after the toner image is heated and melted, it is brought into press contact with the recording medium in the non-heating state, much heat is not taken by a pressure member or the like at the time of heating, so that heating with high efficiency can be performed. Besides, like an apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 11-352804, by using an electromagnetic induction heating device as a heating device, the toner image can be heated up to a predetermined temperature in a short time, and a warm-up time at the time of starting the operation of the device becomes almost unnecessary.
On the other hand, there is widely conventionally used a device which electrostatically performs transfer of a toner image formed on an image carrier onto a recording medium. In such a device, transfer and fixation are not performed at the same time as described above, but after transfer is electrostatically performed, the toner image is fixed by a fixing device. That is, in the case where an image is directly transferred from the image carrier to the recording medium, the transfer is generally electrostatically performed, and thereafter, the toner image is fixed by the fixing device. Also in the case where an intermediate transfer member is used, both primary transfer from the image carrier to the intermediate transfer member and secondary transfer from the intermediate transfer member to the recording medium are electrostatically performed, and a fixing device is provided at the downstream side of the secondary transfer position to obtain a fixed image.
As the fixing device, many devices have been proposed, for example, a device in which a recording medium carrying a toner image is heated and pressed between two rolls having built-in heaters.
As one of them, there is a device in which an endless fixing belt is laid across in a tensioned condition and is circularly driven, and after this belt is heated, it is pressed against a recording medium carrying a toner image by pressure. This device heats the toner image by heat stored by the belt through heating and presses it onto the recording medium, and there is a merit that effective heating can be made by carrying out the heating at the upstream side of a nip portion where pressurization is made.
The above-described image forming apparatus in which secondary transfer and fixation are performed at the same time, and the fixing device in which the heated fixing belt is pressed against the recording medium by pressure have similar problems as described below.
A first problem is such that there is a case where a belt is deformed to corrugate between a position where a belt-like intermediate transfer member or a fixing belt is heated and its downstream nip portion. It appears that this corrugated state occurs since the intermediate transfer member or the fixing belt is heated, so that its portion is expanded, and distortion in the width direction is generated, and tensile force is introduced in the circumferential direction. Especially, in the image forming apparatus in which secondary transfer and fixation are performed at the same time, if the driving speed of the intermediate transfer member is high, heating must be made quickly, so that a low heat capacity thin belt is used, and a corrugated state becomes easy to generate in such a thin belt.
When the belt-like intermediate transfer member comes to have the corrugated state, an image to be transferred is distorted, or permanent wrinkles are produced, and a defect of the image is caused. Also in the case where the corrugated state occurs in the fixing belt, wrinkles which can not be restored are produced on the belt, and poor fixation is caused.
A second problem is such that since heating is not performed at the nip portion after the intermediate transfer member or the fixing belt is heated, poor fixation occurs, or toner remains on the belt, so-called offset can occur.
In general, in a nip portion, at least one of pressed members includes an elastic member, and a predetermined nip length in a circumferential direction is secured by the deformation of the elastic member. Then, the distribution of contact pressure (nip pressure) in this nip is such that the nip pressure is gradually increased from the upstream side in the movement direction of the belt, becomes maximum at a portion near the center, and is gradually decreased toward the nip outlet.
On the other hand, a temperature change when melted toner on the intermediate transfer member passes through the nip is such that when it runs into this nip, heat is absorbed by a recording medium or a pressure member and the temperature is quickly lowered. Thus, a sufficient nip pressure does not act at a portion near the nip inlet where the temperature of the toner is high, and a large nip pressure acts at the nip center portion where the temperature of the toner is lowered. Like this, even if the large pressure acts after the temperature of the toner is lowered and the fluidity is lowered, the toner does not sufficiently permeate the recording medium, and the poor fixation or offset occurs.
In a fixing device, when a recording medium carrying an unfixed toner image passes through a nip, the toner image comes in contact with a heated fixing belt at a nip inlet, and although the temperature is quickly raised to cause a melted state, thereafter, heat is diffused to the recording medium or a pressure member and is quickly decreased. Thus, similarly to the case of the intermediate transfer member, a sufficient nip pressure can not be obtained at a portion where the toner is melted and the temperature is high, and a high nip pressure acts at the nip center portion where the temperature of the toner is lowered. Thus, the poor fixation or offset is caused as well.
The present invention has been made in view of the above circumstances and provides an image forming apparatus which transfers and fixes a toner image on an intermediate transfer member to a recording medium at the same time and in which use efficiency of heat energy is improved and occurrence of image distortion and poor transfer is prevented, or a fixing device which fixes a toner image carried on a recording medium and in which use efficiency of heat energy is improved and occurrence of poor fixation and offset is prevented.
According to an aspect of the present invention, the image forming apparatus includes an image carrier which has an endless circumferential surface and on which a toner image is formed of toner selectively shifted onto the circumferential surface, an intermediate transfer member which is an endless belt shaped member laid across at least two members in a tensioned condition and is circularly driven, and onto an outer peripheral surface of which the toner image on the image carrier is primary transferred, and a transfer fixation device which heats the toner image on the intermediate transfer member, attaches the melted toner to a recording medium by pressure, and performs secondary transfer and fixation at the same time. In the transfer fixation device, a fixed pad is brought into contact with an inner peripheral surface of the intermediate transfer member, the intermediate transfer member is rubbed against a surface of the fixed pad, and a pressure roll presses the intermediate transfer member to the fixed pad to form a nip. A heating device for heating and melting the toner on the intermediate transfer member is provided at an upstream side of a contact position of the fixed pad in a movement direction of the intermediate transfer member, and a corrugation suppressing member for bringing a convex surface into close contact with the inner peripheral surface of the intermediate transfer member is provided between a position where the heating device is provided and a position where the fixed pad is provided.
In the image forming apparatus like this, the toner image formed on the image carrier is primary transferred onto the intermediate transfer member. Then, the toner image, together with the intermediate transfer member, is heated and melted by the heating device. The melted toner image is transported by circulating movement of the intermediate transfer member, is overlapped with a sheet-like recording medium, and is sent to the nip portion between the fixed pad and the pressure roll.
When the intermediate transfer member is heated by the above heating device, a heated portion is thermally expanded, and distortion is produced. Since the intermediate transfer member is circularly driven in a state where tensile force is introduced in the circumferential direction, it is in a state where corrugated wrinkles are easy to produce between the position where the heating device is provided and the nip portion. However, the corrugation suppressing member is brought into contact with the inner peripheral surface of the intermediate transfer member, and the intermediate transfer member is slid in the state where it is brought into close contact with this curved surface. Accordingly, the intermediate transfer member is moved without producing a corrugated state, and occurrence of distortion of the toner image carried by this is prevented.
The toner image sent to the nip is attached to the recording medium by pressure, and is fixed. Together with this, the heat of the toner is absorbed by the recording medium and the pressure roll, and the temperature is quickly lowered. Then, cohesive force of the toner becomes high at the outlet of the nip, and is peeled off from the intermediate transfer member in the state where it adheres to the recording medium and without causing the offset.
As the heating device used in the image forming apparatus, an electromagnetic induction heating device can also be used, in which the intermediate transfer member including a conductive layer is used and eddy current is generated in this layer to make heat generation. Besides, the heating device may be such that a thin metal plate is brought into contact with the intermediate transfer member, eddy current is generated in this metal plate to raise temperature, and the intermediate transfer member is brought into contact with this metal plate and is heated. Further, the heating device may be such that a member having a built-in heater or halogen lamp and coming in contact with the intermediate transfer member, for example, a heat roll is heated to a predetermined temperature, or a member such as a ceramic heater or resistance heat generator is brought into contact with the intermediate transfer member directly or through a member having low friction.
Further, although the fixed pad may be directly brought into contact with the intermediate transfer member and is slid, it is desirable to coat the surface with a layer for reducing abrasion.
The corrugation suppressing member provided between the heating device and the nip comes in contact with the intermediate transfer member heated by the heating device, and it is desirable that the member is made to have low heat capacity so that it does not absorb much heat from the intermediate transfer member. For example, it is desirable to use such a member that the curved surface is formed by working a heat-resistant film of synthetic resin etc., a thin metal plate, or the like. By using such a member, the heated toner image is kept at high temperature to the nip portion, and excellent transfer and fixation are performed. Besides, it is desirable that the corrugation suppressing member is arranged to be continuous with or close to the fixed pad to form a continuous convex curved surface, and sliding of the intermediate transfer member is made smooth.
Besides, it is desirable that the nip is set so that the pressure distribution has a maximum in the vicinity of the inlet. Specifically, there is used the fixed pad which is formed of an elastic material and in which the surface near the inlet of the nip is formed to be convex, the fixed pad which is made of an elastic material and in which a member hard to deform is embedded under the surface near the inlet of the nip, or the like.
Incidentally, the convex curved surface of the fixed pad, the corrugation suppressing member and the heating device can be made one or part of the members across which the endless belt shaped intermediate transfer member is laid in a tensioned condition.
On the other hand, the fixing device of the present invention has the following constitution.
The fixing device of the present invention heats and presses a sheet like recording medium on which a toner image formed by selective shift of toner is transferred, so that the toner image is fixed to the recording medium, and includes an endless fixing belt which is laid across at least two members in a tensioned condition and is circularly driven, a fixed pad fixedly supported to come in contact with an inner peripheral surface of the fixing belt, a pressure roll for interposing the fixing belt and the recording medium overlapped therewith between the pressure roll and the fixed pad to press them, a heating device for heating the fixing belt at an upstream side of a position where the fixed pad is brought into contact with the fixing belt, and a corrugation suppressing member brought into contact with the inner peripheral surface of the fixing belt between a position where heating of the toner image is performed and a position where the fixed pad is provided.
In this fixing device, the fixing belt is heated by the heating device, is overlapped with the recording medium carrying the unfixed toner image, and is sent to a nip portion where the fixed pad and the pressure roll are pressed against each other. In the nip, heat is supplied to the toner image on the recording medium from the heated fixing belt, and the toner image is melted and is attached to the recording medium by pressure.
In the process, the fixing belt heated by the heating device is thermally expanded at this portion, a difference in belt width occurs on the way to the nip, and distortion is produced. However, the corrugation suppressing member is brought into contact with the inner peripheral surface of the fixing belt at this portion. By this, the fixing belt moves in the state where it is in close contact with the corrugation suppressing member, and it is possible to prevent the section from being deformed and to prevent corrugated wrinkles from being produced in the circumferential direction.
As the hating device used in the fixing device, a device having the same structure as the device for heating the intermediate transfer member in the foregoing image forming apparatus can be adopted. A device having a built-in heater or halogen lamp, a device including a ceramic heater, a device performing electromagnetic induction heating, or the like can be used. With respect to the fixing belt as well, one having the same structure as the belt used as the intermediate transfer member described before can be used. Further, the corrugation suppressing member and the fixing pad can also be made to have the same structure.
Incidentally, in the fixing device, with respect to at least the two members across which the fixing belt is laid in a tensioned condition, one is a roll rotated and driven, and the other member is a rotatably supported roll, a fixedly supported pad, or the like. The fixed pad forming the nip can also be made one of the members across which the fixing belt is laid.
Although the above described fixing device is provided with the corrugation suppressing member between the region where the fixing belt is heated and the nip portion, it is also possible to adopt such a structure that the heating device includes a convex curved surface brought into contact with the inner peripheral surface of the fixing belt, and this curved surface is continuous with or close to the contact surface of the fixed pad to the fixing belt so that a continuous convex curved surface is formed.
This adopts in the fixing device the same structure as the structure in the secondary transfer portion of the above described image forming apparatus, in which the convex curved surface of the heating device is continuous with the contact surface of the fixed pad.
Also in the fixing device of such structure, the toner image on the recording medium is melted by heat stored in the fixing belt, and can be attached to the recording medium by pressure, and it is possible to prevent the fixing belt from having the corrugated state and to perform excellent fixation.
Besides, in the fixing device of the present invention, in the nip formed by the press contact of the fixed pad and the pressure roll, the nip pressure at a place near the inlet is made high, so that excellent fixation can be performed without producing poor fixation or offset.
In the nip, the heated fixing belt and the toner on the recording medium come in contact with each other, the temperature of the toner is quickly raised, and the toner is melted. However, thereafter, the heat is absorbed by the recording medium and the pressure roll, and the temperature of the toner is quickly lowered. In the above fixing device, the toner temperature at the place near the nip inlet is high, and the toner is pressed to the recording medium by a large nip pressure when the temperature is at least glass transition temperature of the toner, so that excellent fixation is realized.